1. Field of the Disclosure
The present disclosure relates in general to methods for improving drilling operations, and in particular to a system and method for estimating the strength of rocks during drilling operations. The estimate strength is used in the selection of drill bits, improving drilling performance, and maintaining borehole integrity.
2. Summary of the Related Art
Drill bits do not normally suffer much of any abrasive wear through carbonate formations. The minerals in carbonate rocks are typically much softer than the materials used in bit cutting structures, and so they don't cause significant abrasive wear of the bit cutting structure. Cutting structure degradation does, however, occur in carbonate rocks. This usually involves fracturing of the cutting structure, the scale of which can range from micro-chipping through chipping and spalling to gross fracture. Whatever its scale, this cutting structure degradation will be controlled at least in part by the strength of the rock being drilled. Hence the importance of knowing the strength of the formations to be drilled is important, even in those formations which are not normally abrasive.
On the other hand, in clastic rocks, bit abrasion is an undesirable operating condition which impedes drilling operations. In general, drilling operations are performed in a manner which minimizes the possibility of the occurrence of bit abrasion. One factor which can be controlled is the selection of particular drill bits from a group of available drill bits. Certain bits may be less prone to abrasion under certain drilling conditions, while other bits are more prone to abrasion under certain drilling conditions. The selection is determined based on the strength of the rock.
The strength of the rock is also an important factor in drilling operations such as the selection of weight on bit, torque and RPM. While selection of the mud weight to avoid formation damage and blowout is primarily determined by the formation pore pressure, the strength of the rock also plays an important role in the selection of mud weight required to maintain a stable wellbore. Additionally, selection of casing points is also often based in part upon knowledge of the rock strength. The present disclosure is directed towards a method of determining rock strength from acoustic velocity measurements which recognises the potential importance of grain size in the determination of rock strength.